This invention concerns electronic switches, particularly multiplexers and demultiplexers.
A multiplexer is a circuit with two or more electronic switches that selectively connects one input pin to one of several output pins, and a demultiplexer is a circuit with two or more switches that selectively connects one of several input pins to one output pin. For example, a two-to-one multiplexer includes two switches: one connected between a first input pin and an output pin, and the other connected between a second input pin and the output pin. The two switches are connected also to a control pin which receives a control signal for alternately opening and closing the two switches.
Applying a relatively-high-voltage control signal to the control pin causes the multiplexer to connect the first input pin to the output pin, whereas applying a relatively-low-voltage control signal connects the second input pin to the output pin. A one-to-two demultiplexer, which has one input pin and first and second output pins, works similarly. Because many multiplexers can operate as demultiplexers by simply using their inputs as outputs, the term multiplexer broadly refers to both multiplexers and demultiplexers.
In practice, many multiplexers suffer from transient simultaneous input-output connections which momentarily connect an output pin to, not one, but two input pins. These simultaneous connections stem from delays in opening, or breaking, an existing input-output connection. The delays are inherent to the transistors forming the electronic switches of typical multiplexers. The transistors include structural, or parasitic, capacitances which require charging or discharging to break their input-output connections. The charging and discharging occurs for a period of time during which the associated switch remains closed. Since the switches of the multiplexer typically respond in unison to a control signal, one switch is closing, or "making," a connection while another switch is simultaneously opening, or "breaking," its connection, setting up a race condition between the opening and the closing switches. If the closing switch makes its particular input-output connection before the opening switch breaks its connection, simultaneous connection results.
Simultaneous connections are undesirable because they can damage the multiplexer or circuitry coupled to its inputs or outputs. For instance, in a five-volt disk drive system which uses a two-to-one multiplexer to selectively connect a particular circuit node to either five or zero volts, momentarily connecting both the five-volt and the zero-volt input pins to the output pin short-circuits the power supply, causing a large current spike which could destroy not only the multiplexer but other circuitry in the disk drive.
Thus, as a safeguard against undesirable simultaneous connections, a multiplexer needs a "break-before-make" circuit for breaking one input-output connection before making another input-output connection.